Impedance Heating for Heat Exchanger Water Drainage Channels

ABSTRACT

Embodiments of the invention provide anti-icing systems ( 201 ) for drainage channels ( 203; 205 ) used to remove water run-off during heat pump evaporator coil defrosting cycles during cold weather conditions. The invention employs impedance heating to obviate heat tracing cable, its associated thermal insulation and any protective mechanical coverings.

BACKGROUND OF THE INVENTION

The invention relates generally to the field of heating, ventilation and air conditioning (HVAC) systems. More specifically, embodiments of the invention relate to anti-icing systems for drainage channels that remove water run-off produced during heat pump evaporator defrosting cycles.

A heat pump is a machine which moves heat using a compressor from a low temperature reservoir to a higher temperature reservoir. The pump operates a refrigeration cycle where the refrigerant repeatedly changes state from liquid to vapor, and back to liquid. The refrigerant is condensed to release heat in one part of the cycle and is boiled (evaporated) to absorb heat in another part of the cycle. Heat pumps use outside air as a heat source and must be capable of operating in ambient air temperatures ranging from 14° F. to 50° F. (±10° C.).

Heat pumps use liquid to air heat exchangers (air coils) typically made from copper tubes and aluminum fins. Individual heat exchangers may be arranged in multi-circuits.

A heat pump heat exchanger located outdoors is an evaporator where the refrigerant expands at a saturated evaporation temperature. At air temperatures below 42.8° F. (6° C.) the evaporating temperature in the heat exchanger is approximately 28.4° F. (−2° C.). Any humidity in the surrounding air will condense on the exchanger surfaces and freeze.

To operate continuously at these temperatures, the heat exchanger must be defrosted periodically. Frost will accumulate and reduce airflow through the exchanger. The reduction in airflow minimizes thermal exchange and overall heat pump efficiency. Over time, the heat exchanger may become completely covered in ice.

A defrost cycle usually initiates gas heating to defrost the heat exchanger. One method is where the heat pump refrigeration cycle is reversed using a four-way valve, and hot gas discharged from the compressor is injected into the heat exchanger. The hot gas heats the heat exchanger and melts the ice accumulated on the tubes and fins.

During a defrosting cycle, many gallons of water may be released. For example, ice melted from a coil having a frontal area of approximately 21.5 ft² (2 m²) can result in more than 4 gallons (10 litres) of water. Taking into account the quantity of water from each heat exchanger per defrost cycle, it is necessary to channel and remove the water to the outside of the unit. If not, water present on the interior surfaces of a drainage channel will gradually freeze and build-up, blocking flow in each channel. The blocked flow may result in damaged components located within the evaporator housing.

Drainage channels having a slope angle of several degrees are typically provided and guide the melted water outside of the housing. To prevent the water from freezing in the channels, the channels are usually heated and insulated.

A typical heat pump evaporator system 101 is shown in FIG. 1. A pair of heat exchangers 103 and 105 are oriented at an acute angle with respect to each other and converge away from two circulation fans 107, 109. The fans 107, 109 are supported by the coils 103, 105, additional support members 111, 113 and fan motor support brackets (not shown). Solid covers 115 are positioned on the ends and around the circulation fans 107, 109 such that the circulation fans 107, 109 draw air through the exchangers 103, 105. Two drainage channels 117, 119 are provided to collect and guide water run-off 121 from the coils during defrosting cycles. To prevent drainage channel 117, 119 freezing when outside temperatures are at or below freezing, heat tracing 123 may be installed running under the length of each channel or wrapped around each channel.

Typical heat tracing cable has two conductors inside a cable insulation which run from end to end and are separated from each other by a carbon type material which is conductive and produces the heat. Heat tracing cable is usually rated in Watts (power dissipation) per unit length.

After heat tracing cable is applied, it is usually thermally insulated and mechanically covered for protection. Heat tracing cable is susceptible to damage and may represent a health risk for people due to the elevated voltages present (230-400 Vac). Heat tracing is expensive to install and requires protection against electric shocks.

It is desirable to develop a channel drainage heating system that is simple and effective, and eliminates thermal and mechanical covers.

SUMMARY OF THE INVENTION

Although there are various systems that are available to heat HVAC drainage channels during cold weather conditions, such systems are not completely satisfactory. The inventors have discovered that it would be desirable to have systems that heat drainage channels used to remove water run-off during heat pump evaporator coil defrosting cycles using impedance heating. Impedance heating obviates heat tracing cable and its associated thermal insulation and mechanical covers.

One aspect of the invention provides a drainage channel heating system. Systems according to this aspect of the invention comprise a first terminal electrically coupled to a drainage channel at a first location and a second terminal electrically coupled to a drainage channel at a second location, a resistance defined between the first and second terminals, a transformer having a primary winding having two connections and a secondary winding having two connections, a loop defined by the first drainage channel terminal coupled to one secondary connection and the second drainage channel terminal coupled to the other secondary connection, a predetermined secondary voltage at the secondary connections is present when a predetermined primary voltage is impressed on the primary connections, and a loop current is present when the primary voltage is impressed on the primary connections wherein the loop current is determined by the predetermined voltage and resistance.

Another aspect of the invention provides a drainage channel heating system. Systems according to this aspect of the invention comprise a conductive strip sized in matching correspondence with an interior of a drainage channel, a first terminal electrically coupled to a conductive strip at a first location and a second terminal electrically coupled to a conductive strip at a second location, a resistance defined between the first and second terminals, a transformer having a primary winding having two connections and a secondary winding having two connections, a loop defined by the first strip terminal coupled to one secondary connection and the second strip terminal coupled to the other secondary connection, a predetermined secondary voltage at the secondary connections is present when a predetermined primary voltage is impressed on the primary connections, and a loop current is present when the primary voltage is impressed on the primary connections wherein the loop current is determined by the predetermined voltage and the resistance.

Yet another aspect of the invention provides a drainage channel. Drainage channels according to this aspect of the invention comprise a U-shaped structure having a bottom and two side walls, a conductive material embedded within the drainage channel structure, and a first terminal at a first end of the drainage channel and a second terminal at a second end of the drainage channel wherein said first and second terminals conductively couple with said conductive material.

The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary perspective view of an evaporator system.

FIG. 2 is an exemplary embodiment of the invention.

FIG. 3 is an alternative embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of the invention will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout. Further, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. The terms “mounted,” “connected,” and “coupled” are used broadly and encompass both direct and indirect mounting, connecting, and coupling. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings.

Embodiments of the invention provide anti-icing systems for drainage channels used to remove water run-off during heat pump evaporator coil defrosting cycles during cold weather conditions. The invention employs impedance heating to obviate heat tracing cable, its associated thermal insulation and any protective mechanical coverings.

Direct impedance heating involves passing an alternating current through a material to be heated. The resistance of the material to the current being passed through it generates heat. Low frequency current, such as 60 Hz is used to heat the material. High frequency current, such as 400 Hz may be used to heat just the surface of the material.

The resistance of the material to be heated cannot be too low or a high current may be necessary to reach a desired power level. Similarly, the resistance may not be too high or a high voltage will be necessary as witnessed using Ohm's law,

V=IR,  (1)

where V is Voltage, I is current in Amps and R is resistance in Ohms. By using a low voltage, for example ≦6 Vac, the drainage channels may be direct impedance heated.

An isolation transformer having a high power (VA) rating is used to provide a low secondary voltage to the drainage channels. The transformer decouples and isolates the transformer primary winding from the transformer secondary winding.

A transformer allows alternating current to be supplied to a device from a source without a common electrical connection between the two circuits. The transformer has primary and secondary windings which are not electrically coupled to one another. The windings are coupled by magnetic effects in the iron core. Since the neutral connection of the ac source is directly connected to earth (ground), grounded objects near the load coupled to the secondary windings may be at a hazardous potential difference with respect to the load. By using a transformer, the common coupling between the two neutrals is eliminated and any shock hazard will be contained (isolated) in the load. To avoid shock hazards, each drainage channel may be grounded to the unit frame.

The transformer power dissipation is usually expressed in VA (Volts-Amps). The transformer used for the preferred embodiment is rated at 475 VA and is coupled to a 400 Vac mains supply. The transformer construction may comprise a plurality of stacked laminations or one continuous lamination (toroid).

The number of windings is determined by,

$\begin{matrix} {{V_{1} = \frac{V_{2}}{n}},} & (2) \end{matrix}$

where V₁ is the primary side voltage, V₂ is the secondary side voltage and n is the turns ratio

$\frac{N_{2}}{N_{1}}.$

The turns ratio in the exemplary embodiment is defined to output a predetermined voltage in the range of from greater than 0 Vac to less than or equal to 6 Vac. 6 Vac is preferred in view of the safety standards for protecting against electric shock when in high humidity environments or if immersed in water. The secondary winding N₂ may only comprise several turns.

Shown in FIG. 2 is a first embodiment 201 of the invention. The embodiment 201 employs two drainage channels, a first channel 203 and a second channel 205, one for each evaporator exchanger assembly (not shown) or assemblies, each having a substantially U-shaped cross section. Other drainage channel cross sections such as V-shaped may be used. An evaporator exchanger assembly may comprise a plurality of individual exchangers.

Forward exchanger end caps 207, 209 are shown in phantom for exchanger position reference. Exchanger assembly base plates 211, 213 are positioned such that water run-off during defrosting cycles may cascade down through the tubes and fins to the base plates 211, 213 and spill into each evaporator assembly's respective drainage channel 203, 205. Each drainage channel 203, 205 is positioned at the edge, or under a lip of the base plate 211, 213, and mounted at an angular slope with regard to level to allow collected water to seek a lower level.

The invention may be practiced with evaporator exchanger systems having one drainage channel, as well as evaporator exchanger systems having a plurality of individual drainage channels. The invention may be adapted for any number of drainage channels and configurations.

The drainage channels 203, 205 employed in this embodiment are constructed of galvanized steel for strength, and corrosion and rust resistance. The drainage channels 203, 205 are electrically insulated from each other and all exposed framework by using insulating hardware between each channel-to-frame mounting point except where grounded. The insulating hardware may comprise washers, fasteners, or pads made of nylon, plastic, rubber, phenolic or other electrically insulating material. The electrical resistance of steel is 17.4×10⁻⁶ Ωcm and the electrical resistance of zinc is 5.9×10⁻⁶ Ωcm.

In an alternative embodiment, the drainage channels 203, 205 may be made of a predetermined carbon/plastic composite having a known electrical resistivity per unit length. In another alternative embodiment, a metal sheet is embedded in the bottom of a drainage channel made of a non-conducting material 203, 205 at the time of molding or extrusion. The metal sheet may be planar, or generally of a U-shaped configuration sized to match the configuration of the drainage channel. For coupling with electrical conductors, the internal metal sheet may be accessed using rivets, sheet metal screws, or other penetrating hardware.

At least two terminals 215, 217, 219, 221 are electrically coupled to each drainage channel 203, 205. A first terminal 215, 217 is positioned at a first channel end and a second terminal 219, 221 is positioned at a second channel end. The terminals for each drainage channel are located on a channel such that the maximum, localized channel heating is at the bottom of the channel where the greatest likelihood of freezing will be experienced. In the same manner, the width of a channel may be sized to increase bottom heating. The terminals may be screw-type terminals, spade-type terminals, ring-lug type terminals or others, for fastening onto a welded, threaded stud, or tapped hole.

If more than one drainage channel is used for an evaporator exchanger system, the channels are coupled together as an electrical series circuit where the second terminal of one channel is coupled to the first terminal of another channel. The two drainage channels 203, 205 are coupled together using a single conductor cable 223 coupled to the second terminal 219 of the first channel 203 and to the first terminal 217 of the second channel 205. The common coupling 223 may be grounded. The first terminal 215 of the first channel 203 and the second terminal of the second channel 205 are coupled to secondary windings 225 of a transformer 227 using two single conductor cables 229, 231. The circuit arrangement of the two drainage channels 203, 205 and secondary winding 225 constitutes a low voltage secondary loop in which a high current flux (Amps) circulates. The primary winding 233 is coupled to a 400 Vac source (not shown).

In the preferred embodiment, a voltage of 1.8 Vac and a current of 280 A is delivered at the secondary transformer taps. The power dissipated in the drainage channels is,

P=IV.  (3)

Substituting,

P=(280A)1.8V,  (4)

P=504VA.  (5)

The total load is 504 VA. Both channels 203, 205 used in the preferred embodiment are of equal size, with each drainage channel 203, 205 dissipating approximately 252 VA.

The current through the channels provides the necessary heating to allow the heat pump to be operated continuously during cold weather despite frequent defrosts that may last upwards of one half hour. After more than 15 hours of continuous operation, no phenomenon of localized ice build-up in the drainage channels was observed at a low ambient temperature of 14° F. (−10° C.) and a high relative humidity of >90%. The galvanized steel drainage channels were without insulation.

The mains voltage 233 supplied to the transformer 227 may be interrupted using a thermostat contact (not shown) or temperature controller (not shown) when the outside temperature is above a predetermined set point. The mains voltage may also be applied continuously.

Shown in FIG. 3 is a second embodiment 301 of the invention applied to the same heat exchanger assembly configuration. Instead of directly heating the drainage channels 303, 305, heat is applied by direct impedance heating of two insulated metal strips 307, 309 disposed at the bottom of each drainage channel 303, 305. The first 307 and second 305 strips may be galvanized steel, stainless steel, a conductive composite, or other conductive metal dimensioned to and in conformance with each channel bottom. The bottom surfaces of the strips 307, 309 are electrically insulated from each channel 303, 305. Insulating material having a high electrical resistance but low thermal resistance is used, such as conformable high-strength foams (3M VHB tape) or others.

At least two terminals 311, 313, 315, 317 are conductively coupled to each heating strip 303, 305. A first terminal 311, 313 is positioned at a first strip end and a second terminal 315, 317 is positioned at a second strip end.

If more than one drainage channel is used for an evaporator exchanger system, the strips are coupled together as an electrical series circuit where the second terminal of one channel is coupled to the first terminal of another channel. The two strips 303, 305 are coupled together using a single conductor cable 223 coupled to the second terminal 315 of the first strip 303 and to the first terminal 313 of the second strip 305. The first terminal 311 of the first strip 303 and the second terminal 317 of the second strip 305 are coupled to secondary windings 319 of a transformer 321 using two single conductor cables 229, 231. The circuit arrangement of the two strips 307, 309 and secondary winding 319 constitutes a low voltage secondary loop in which a high current flux (Amps) circulates. The primary winding 233 is coupled to a 400 Vac source (not shown).

A voltage of 4.0 Vac is delivered by the secondary winding with a 108 A secondary current. Total power dissipated through the heating strips 303, 305 is 432 VA.

The advantages of the invention are increased system safety for service personnel due to the use of very low voltage, increased system reliability since the only component susceptible to failure is the transformer, no heaters to become unfastened from the channels or degraded due to UV exposure and no significant maintenance of the system is required on site.

One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims. 

1. A drainage channel (203; 205) heating system comprising: a first terminal (215; 217) electrically coupled to a drainage channel at a first location; a second terminal (219; 221) electrically coupled to a drainage channel at a second location, said first and said second terminals configured to define a resistance; and a transformer (227) having a primary winding and a secondary winding (225), said secondary winding (225) coupled between said first terminal and said second terminal, wherein a predetermined secondary voltage is present at said secondary winding (225) when a predetermined primary voltage is impressed across connections of said primary winding (233), said predetermined secondary voltage for effecting a current in a drainage channel determined by said predetermined secondary voltage and said resistance.
 2. The system according to claim 1 wherein said predetermined secondary voltage is in the range of from greater than 0 Volts to less than or equal to 6 Volts.
 3. The system according to claim 2 wherein said drainage channel (203; 205) is electrically insulated.
 4. The system according to claim 3 wherein said drainage channel is constructed of galvanized steel.
 5. The system according to claim 4 wherein said first terminal (215) and said second terminal (219) are located on the same drainage channel (203).
 6. The system according to claim 4 wherein said first terminal (215) and said second terminal (221) are located on different drainage channels (203; 205).
 7. The system according to claim 6 wherein said predetermined secondary voltage is determined by a turns ratio.
 8. The system according to claim 7 wherein said predetermined primary voltage (233) is determined based on said turns ratio and said predetermined secondary voltage.
 9. The system according to claim 8 wherein the drainage channel heating system is for a heat pump evaporator assembly (101).
 10. A drainage channel (303; 305) heating system comprising: a conductive strip (307; 309) sized in matching correspondence with an interior of a drainage channel (303; 305); a first terminal (311; 313) electrically coupled to a conductive strip at a first location; a second terminal (315; 317) electrically coupled to a conductive strip at a second location, said first and said second terminals configured to define a resistance; and a transformer (321) having a primary winding and a secondary winding (319), said secondary winding (319) coupled between said first terminal and said second terminal, wherein a predetermined secondary voltage is present at said secondary winding (319) when a predetermined primary voltage is impressed across connections of said primary winding (233), said predetermined secondary voltage for effecting a current in a conductive strip determined by said predetermined secondary voltage and said resistance.
 11. The system according to claim 10 wherein said predetermined secondary voltage is in the range of from greater than 0 Volts to less than or equal to 6 Volts.
 12. The system according to claim 11 wherein said conductive strip (307; 309) is electrically insulated.
 13. The system according to claim 12 wherein said conductive strip (307; 309) is coupled to an interior of a drainage channel (303; 305).
 14. The system according to claim 13 wherein said first strip terminal (311) and said second strip terminal (315) are located on the same strip (307).
 15. The system according to claim 13 wherein said first strip terminal (311) and said second strip terminal (317) are located on different strips (307; 309).
 16. The system according to claim 15 wherein said predetermined secondary voltage is determined by a turns ratio (319).
 17. The system according to claim 16 wherein said predetermined primary voltage (233) is determined based on said turns ratio and said predetermined secondary voltage.
 18. The system according to claim 17 wherein the drainage channel heating system is for a heat pump evaporator assembly (101).
 19. A drainage channel comprising; a U-shaped structure having a bottom and two side walls; a conductive material embedded within said drainage channel structure; and a first terminal at a first end of the drainage channel and a second terminal at a second end of the drainage channel wherein said first and said second terminals conductively couple with said conductive material.
 20. The drainage channel according to claim 19 wherein said conductive material is a metal.
 21. The drainage channel according to claim 20 wherein said conductive metal is flat and sized in correspondence with said drainage channel bottom.
 22. The drainage channel according to claim 20 wherein said conductive metal is U-shaped and sized in matching correspondence with said drainage channel bottom and sidewalls.
 23. The drainage channel according to Claim 19 wherein said conductive material is a conductive plastic.
 24. The drainage channel according to claim 23 wherein said conductive plastic is said U-shaped structure. 